When ventilation of an anaesthetised patient is required during surgery a tube is attached from a mechanical ventilator to the patient's mouth or trachea. Ventilators performing this function are typically equipped with a number of safety features to check whether the patient is properly ventilated. One form of testing relies on measuring the pressure of the gas supplied to the patient.
Dysfunction of the ventilator includes sensing when too high a supply pressure is delivered, too low a supply pressure is available, or there is an absence of the expected cyclic pressure pattern of a typical ventilation cycle. The cyclic pattern is determined by measuring whether the pressure excursions traverse a set level within a given (inspiration) period. Once an unsafe condition has been detected an alarm of some type is activated. A delay is generally employed to obviate a false alarm. In the sensing of the absence of the cyclic pattern a delay of from fifteen to twenty five seconds may be employed while in the case of the sensing of a high or low pressure a delay of typically one second is employed before the alarm is activated.
Due to the variability in the circumstances during surgery and considering individual differences between patients the fixing of the pressures at which the alarms will activate in either the low or high pressure conditions is set by the attending physician upon observing the pressure variation of the patient during a ventilation cycle.
Notwithstanding these features a loss of intubation of the patient may not be detected until after the appropriate delay has passed or may not be detected at all if the window defining the limits of upper and lower pressure specified by the attending physician is inappropriately adjusted. The disconnected line may still show the required cycling within the stipulated bounds and no alarm would be indicated.
A further safety feature during anaesthesia is the provision of capnography, measuring the amount of carbon dioxide in the exhaled breath of the patient. In the absence of sufficient carbon dioxide being measured an alarm is sounded after a delay of fifteen to twenty seconds. Other monitors are also used to check for the signs of proper ventilation of a patient, for example a pulse oximeter measures the oxygen in the blood of a patient's finger employing an optical technique but involves an even longer delay before sounding an alarm.